If you ride bikes or run sleds, you already know that the sound of a two-stroke, triple cylinder is like a banshee wailing across the lake or down the highway.
But that’s not why AchatesPower went with the two-by-three formula. They did it because physics dictates that the number of cylinders makes a big difference in the performance and efficiency of an opposed-piston, two-stroke engine, and many other engines as well.
AP determined that three-cylinders is the best design in terms of gas-exchange, when compared to a two-cylinder or four-cylinder design. The gas exchange duration in a two-stroke engine is 120 degrees crank angle. In a three-cylinder design, the scavenging events are aligned to have minimal interference with each other.
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The three-cylinder OP two-stroke configuration has slightly overlapping gas-exchange events, resulting in a phenomenon known as cross charging.
In a two-cylinder configuration, gas-exchange events are too far separated. In a turbo charged application, this separation causes the turbo to lose energy over the cycle, which has a negative effect on efficiency, especially at lower loads and low engine speeds. This loss of turbocharger energy has to be compensated by a crank-driven supercharger, which causes a reduction in brake thermal efficiency.
Conversely, in a four-cylinder configuration, the gas-exchange events overlap too much. This causes cross charging to occur at a point in time when hot exhaust gases are leaving the cylinder. The interruption of exhaust gas flow causes an increase in residual gas content and a lower scavenging efficiency which leads to a reduction in power.
